Traditionally, paging encoders have been bulky, expensive, desk bound units suffering from limited capacity and a lack of adaptability which would allow operation across the spectrum of different selective signalling formats.
Incompatibility between signalling formats of the various paging systems creates a severe handicap on prior art systems because numerous formats are currently employed. The most popular are: the Motorola two-tone, Motorola five-tone, Motorola six-tone, General Electric two-tone (Type 99), and Reach two-tone signalling formats.
Attempts have been made to provide paging encoders which are capable of operating within the requirements of the various signalling formats, but such systems have been expensive to design and fabricate due to the numerous differences between the various formats.
For example, Motorola utilizes one second for the first tone, two to three seconds for the second tone, and requires seven to eight seconds for group calls. The system employs six reed groups of ten tones each, which permit about 870 codes in the basic tone pairing scheme and approximately 3,500 tone pairs in an extended assignment plan.
General Electric employs a 1-second first tone and a 1.5-second second tone, and does not allow for any group call. Instead it provides an extra tone, known as the diagonal tone, to replace the first tone of any identical tone pair. This occurs in pager codes employing a 0, 2, or 4 as the first number of the cap code. Incorporated in their code plan are three reed groups of ten tones each plus one diagonal tone that allows them to generate 900 different codes.
The Reach system features both fast and slow two-tone sequences. They have a wider frequency spread between their tones to facilitate high-speed encoding. Their high-speed tone timings are running about 100-150 milliseconds for both tones. The slower format employs a 2-second first tone and a 700-millisecond second tone. This slower scheme works in conjunction with their battery-saving feature to permit up to a year's operation between battery changes. Reach uses a single tone of 5 seconds duration to initiate group call. The Reach format incorporates a total of 60 tones. However, only tones 11-55 are used for two-tone selective calling and that permits 1000 codes.
Motorola uses a totally different strategy in its five- and six-tone decimal digital pager. Rather than selecting two tones from a large range of frequencies to generate all of the tones needed in a high capacity system, Motorola elected to use a new technique that allows the pagers to generate from 100,000 to 1,000,000 codes using only 12 tones. The straight five-tone address will produce 100,000 calls while adding 10 different preambles ahead of each address would accommodate 1,000,000 codes. This 1,000,000 code capacity would apply only to pagers with the battery-saving option that relies on the correct preamble to wake it up.
The basic signalling scheme used in the five-tone sequence consists of an optional 690 millisecond preamble tone followed by a 45-millisecond gap of unmodulated carrier, then five sequential tones each of 33 milliseconds in length and either a 52-millisecond gap (five-tone) or 52 milliseconds of special tone X (six-tone).
The X tone is used to activate the uninterrupted tone in the dual-address pagers instead of the normal pulsating tone that results from a five-tone address. Twelve frequencies are used to represent the digits 0-9, repeat tone R, and special tone X. The repeat tone is substituted each time there are two identical, successive digits appearing in the address code. For example, an address code of 25597 would be converted to 25R97. The preamble can be set to any one of the 10 tones 0-9, or can duplicate the first number in the address code.
In addition to all the different format problems, it is difficult to make any code assignment changes within the same format. For example, in the Motorola expanded code assignment plan, there are over 20 different cap code prefixes producing 180 group combinations (Motorola Pager Manual Table 3). Most encoders are capable of handling only one of the 20 combinations at a time, if that many. In earlier units, it was necessary to change reed banks in order to change from one code assignment to another.